Already in 1980 A. Herz had discovered so-called endorphins, i.e. body-own (endogenous) morphine, which binds at special opiate receptors of the brain. After 1988 also the proof of special cannabinoid receptors succeeded, whose structure was cleared up 1990 (CB1) and 1992 (CB2). 1992 with anandamide a first endogenous cannabinoide was discovered, which 1995 the 2-arachidonoylglycerol (2-AG) followed.
2002 Mayer et al. give an overview to the biological - especially the molecular - bases of the opiate dependence. On the basis of a literature analysis and own investigations, in particular due to animal experiments they determined the neural bases of the dependence process as follows: "The euphoria effect of morphine is based on an activation of the basal front brain bundle in such a way specified, which is regarded as cerebral representation of the reward system. For the emergence of dependence above all its characteristic might be responsible to cause long continuing sensitization in the limbic and mesolimbic brain areas. The latter are interconnected with the hippocampus formation, their meaning for learning and memory procedures are established closely."
Thus by the example of cocaine it was reported that the hippocampus appears responsible for the so-called dependence memory and the associated strong relapse inclination of abstinent former dependents. This phenomenon arises likewise with morphine. It shows up that a strong genetic arrangement exists for the dependence susceptibility, which is based partly on polymorphisms in the genes of the opioid receptors. The authors draw from this fact the conclusions that the development of an opiate dependence proceeds from an appropriate arrangement and represents a continuous process, with which the response behavior of the brain becomes independent and thus changes. As trips for the opiate consumption stress situations are often found, whereby morphine is able to antagonize the reactions of the brain to stress attractions and thus the self-dynamics described above releases. As therapeutic recommendation "an as early as possible intervention in the cascade-like process of the dependence emergence" appears meaningfully to the authors.
Likewise the WHO lexicon describes "neuro-adaptation" as "the neuronal changes associated with both tolerance and the appearance of a withdrawal syndrome. It is possible for an individual to exhibit neuro-adaptation without showing the cognitive or behavioral manifestations of dependence. For example, surgical patients given opiate substances to relieve pain may sometimes experience withdrawal symptoms but may not recognize them as such or have any desire to continue taking drugs."
Lit: P.Mayer, M.Erdtmann Vourliotis, V.Hoellt: Molecular bases of the opiate dependence. Sucht 48 (1):17-28, 2002